This invention relates to an image fixing device for fixing electrophotographic toner images on a substrate such as paper and an electrophotographic apparatus incorporated with such a fixing device.
Various methods are known for fixing a toner image formed by electrophotography on a substrate such as paper or plastic film in the electrophotographic image processing works. Typical and well known among such fixing methods is the method which makes use of heat energy. This method for fixing toner on a substrate by heat requires heating up to a temperature at which the binder resin in the toner is fused or softened to become adhesive. The toner is caused to adhere onto the substrate by the fused and adhesive binder resin. Upon cooling thereafter, the binder resin is solidified and the toner is firmly stuck on the substrate.
The most general method for effecting fusion adhesion of the toner on the substrate is to let the substrate having a toner image formed thereon be passed between a heated roll, which can be heated internally or externally, and a support roll designed to press the toner applied side of the substrate against said heated roll. Used as the heated roll in said fixing device is a roll comprising a rigid core and a fluorine resin coat formed around said core. As for support roll, there is known a roll comprising a rigid core, a layer of elastic material such as organosiloxane rubber attached to said rigid core and a fluorine resin coat on said elastic material layer (Japanese Patent Publication No. 58-43740). There is also known a support roll having a heat resistant elastic material layer and a heat resistant porous elastic body provided on said layer (Japanese Patent Publication No. H1-36624).
In said fixing device, a heated roll is contacted with the toner applied side of a substrate having a toner image formed thereon, and the toner is heated and fixed on the substrate surface while the toner is passed through a nip formed between said heated roll and a support roll.
Said fixing device is usually so constructed that the support roll is pressed by the heated roll so that a recession is formed in the support roll surface by the heated roll at the nip for securing fusion adhesion of the toner image onto the substrate. Therefore, the support roll is composed of a rigid core and a relatively thick elastic material layer formed on said core, and arrangement is made such that said recession is formed continuously always at the nip even through the support roll rotates.
In the conventional fixing devices, because of their structural arrangement designed such that a recession would be formed in the support roll by the heated roll as mentioned above, there was much risk of the substrate being wrinkled or rucked up at the end especially when the substrate was e.g., cardboard such as postcard, or envelope (i.e., a laminate composed of two or more sheets of paper which are joined at the end).
According to the studies by the present inventors, in the case of a substrate such as an envelope for instance, occurrence of said wrinkling is considered originating in the difference in movement of the nip, which is caused due to the difference in linear velocity between front paper and back paper of the envelope at the nip, which in turn is caused because, as shown in FIG. 1, there is a difference, although slight, between the distance 4 from the center of heated roll 1 to front paper 2 of the envelope and the distance 5 from said roll center to back paper 3 of the envelope. Especially when the pressing force of support roll 6 against heated roll 1 is large, as shown in FIG. 2, said support roll 6 is deflected relative to the axis of the roll, producing a difference in pressure between the end portions 9, 9' and the central portion 10 of the roll, giving rise to a difference in nip width 11 between the end 9, 9' and central 10 portions of the roll as shown in FIG. 3. It is considered that this results in producing a difference in movement between the central and end portions of the toner substrate to cause wrinkling of the substrate at its end.